1. Field of the Invention
The present invention relates to stepping motors such as rotary stepping motors and linear stepping motors, and more particularly to driving method thereof wherein stop positioning accuracy is improved.
2. Description of the Prior Art
Stepping motors such as linear stepping motors, rotary stepping motors, etc., are adapted in general to move a primary side magnetic flux generating unit or a secondary side magnetic member by making use of a pulse current supplied to the primary side magnetic flux generating unit, and are capable of precisely controlling the amount of movement thereof (or the amount of rotation thereof). They have therefore been profitably employed for various kinds of precision apparatuses. These motors, however, suffer from some problems. Namely, the accuracy of stopping a movable member of a stepping motor is particularly severe in various applications. In the following discussion, the general arrangement of a stepping motor will first be described, prior to the discussion of these problems.
Referring hereto FIG. 16, illustrating a linear stepping motor as an exemplary prior art stepping motor, the numeral 1 is a secondary side scale (secondary side magnetic member) and the numeral 2 is a movable member (primary side magnetic flux generating unit) placed on the secondary side scale 1.
The secondary side scale 1 described above has a comb-shaped tooth part 1a formed on the upper surface thereof in a prescribed pitch (distance between the centers of teeth adjacent to each other) and guide rails 1b are respectively mounted on both sides of the scale 1.
The movable member 2 described above consists of iron cores 3 and 4 and side plates 5 and 6 for fixing widthwise the iron cores 3 and 4, and magnets 7 and 8 fixedly mounted on the upper parts of the iron cores 3 and 4 and coils 9a, 9b, 9c, and 9d, respectively mounted on respective magnetic poles 3a, 3b, 4a, and 4b of the iron cores 3 and 4 and shafts 10 and 11 mounted on the lower ends of the side plates 5 and 6, and bearings 12 . . . mounted on both ends of these shafts 10 and 11.
Pole teeth 13a, 13b, 14a, 14b are respectively formed on the lower ends of the magnetic poles 3a, 3b, 4a, 4b in the same pitch as that of the tooth part 1a. These pole teeth 13a, 13b, 14a, 14b are opposed to the tooth part 1a of the scale 1 respectively differing by 1/4 pitch from each other. For example, when the pole tooth 14b agrees with the tooth part 1a, the pole tooth 13a is displaced to the right side in the Figure by 1/4 pitch with respect to be tooth part 1a, and likewise the pole tooth 14a is displaced by 2/4 pitch and the pole tooth 13b is displaced by 1/2 pitch.
Next, the operation of this linear stepping motor will be described with reference to FIG. 17. In the figure, the arrow M shows a magnetic flux from the magnets 7 and 8.
The figure shows a state of the linear stepping motor wherein pulse current conduction to the coils 9a and 9a is finished (a magnetic flux is produced in the direction of the arrow P), and a pulse current is conducted through the coils 9c and 9d to generate a magnetic flux shown by the arrow Q. In this situation, the magnetic flux Q is directed in the opposite direction to that of the magnetic flux M from the magnets 7 and 8 on the magnetic pole 4a side to cancel magnetic force of the latter, while it is directed in the same direction as that of the magnetic flux M on the magnetic pole 4b side to enhance the latter. Thus, an attraction force is produced between the tooth part 1a of the scale 1 and the pole tooth 14b of the magnetic pole 4b displaced to the right by 1/4 pitch with respect to the tooth part 1a to, whereby the entire movable member 2 is advanced by 1/4 pitch in the F direction (the figure illustrates a state of the movable member after the movement thereof).
Next, by conducting a pulse current through the coils 9a and 9b after stopping the current conduction to the coils 9c and 9d to generate a magnetic flux in the opposite direction to that of the arrow P, this magnetic flux allows its magnetic force to cancel that of the magnetic flux M on the magnetic pole 3b side while allowing the same magnetic force to enhance the latter magnetic force on the magnetic pole 3a side. Thus, an attraction force is produced between the pole tooth 13a of the magnetic pole 3a and the tooth part 1a of the scale 1 to permit the movable member 2 to again advance in the F direction by 1/4 pitch.
The movable member 2 can be driven to an arbitrary position and stopped at that position by properly altering the direction of the current flow supplied to the respective coils 9a to 9d, and the order thereof in the same manner as the above procedure.
Furthermore, substantially the same principle as in the above linear stepping motor can also be applied to a rotary type stepping motor, and hence the description thereof will be omitted for the sake of brevity.
Such a prior art stepping motor encounters the following problems upon effecting accurate positioning control.
This problem will be described with reference to FIG. 17 again. This figure illustrates as described previously the state of the motor wherein after the pulse conduction to the coils 9a, 9b (the magnetic flux P is generated) is finished, a pulse current is conducted through the coils 9c and 9d to generate a magnetic flux Q.
The magnetic flux P ought to completely disappear in essence in the above situation since the current conduction to the coils 9a and 9b has already been finished. However, the corresponding portions of the iron core 3 and the scale 1 are slightly magnetized by the magnetic flux P generated by the coils 9a and 9b, and hence a weak magnetic flux is left behind as shown by the chain line in the figure (residual magnetic flux). This residual magnetic flux cancels the magnetic flux M on the magnetic pole 13a side while strengthening the magnetic flux M on the magnetic pole 13b side, whereby it produces an attraction force between the pole tooth 13b of the magnetic pole 3b and the tooth part 1a of the scale 1. This attraction force finally acts to pull the movable member 2 to the right. Accordingly, when the movable member 2 is contrived to stop at a prescribed position, the movable member 2 is obliged to stop at a position where the pulling attraction force to the left produced between the pole tooth 14b and the tooth part 1a is adapted to balance the above-mentioned pulling attraction force to the right. Thus, the stop position of the movable member is shifted as shown by H in the figure. A hysteresis is defined as changes in the magnetization of a substance logging behind changes in the magnetic field as the magnetic field is varied. Such a hysteresis is produced at all times when the movable member 2 stops to result in the inaccurate stop positioning of the movable member 2. Such a problem of hysteresis is also encountered by other types of stepping motors.
Furthermore, it is also possible to eliminate such hysteresis by forming the iron cores 3 and 4 with a high-quality magnetic member and magnetically anneal them with scrupulous care, but this method is very costly and, even if executed, can not prevent such a residual magnetic flux from being produced to some degree.
Next another problem encountered by such a motor will be described.
FIG. 18(a) is a schematic illustrating the arrangement of a prior linear stepping motor. As shown in the figure, a movable member I' consists of exciting coils A' and B', cores C' and D', and a permanent magnet E. Symbols P1 to P4 denote magnetic poles. K' is a stator having teeth 1', 2' . . . formed in an equal pitch on the upper surface thereof. FIG. 19 is a block diagram of a driving circuit for driving the coils A' and B' described above. In the figure, elements 1a', 1b', 2a', 2b', 3a', 3b', 4a', and 4b' are respective transistors.
When moving the movable member 1' in the direction of the arrow Y1' of FIG. 18 in the linear pulse motor arranged as such, the transistors 1a', 1b' . . . are driven in the following order: EQU 1a', 1b'.fwdarw.3a', 3b'.fwdarw.2a', 2b'.fwdarw.4a', 4b'.fwdarw.1a', 1b'.fwdarw. . . . .
When the transistors 1a' and 1b' are switched on, a current is forced to flow through the coil A' in the direction of A1'.fwdarw.A2' whereby the movable member I' is moved in the direction of the arrow Y1' and is stopped while allowing the magnetic pole P1' to face a tooth 1' for example, as shown FIG. 18(a). When in the transistors 1a' and 1b' are switched off and the transistors 3a' and 3b' are being switched on, a current is conducted in the direction of from B1' to B2' through the coil B', whereby the movable member I' is further moved in the direction of the arrow Y1' by 1/4 pitch and stopped with its state where the magnetic pole P2' faces to the tooth 4' as shown in FIG. 18(b). The same operation will thereafter be repeated.
Furthermore, when the movable member I' is moved in the direction of the arrow Y2, the transistors 1a and 1b are driven in the following order: EQU 4a', 4b'.fwdarw.2a', 2b'.fwdarw.3a', 3b'.fwdarw.1a', 1b'.fwdarw.4a', 4b'.fwdarw. . . . .
An application of such a linear stepping motor, for example, to drive a magnetic head of a floppy disk drive, requires a positioning accuracy on the order of a micrometer. However, a prior linear stepping motor of the type described above produces a hysteresis phenomenon because of the cores C' and D' having the residual magnetism, in which the movable member I', as subjected to reciprocation, is obliged to stop at different positions when it goes ahead and back.
In the following, the hysteresis will be described with reference to FIG. 21. As shown in the figure, the movable member I', when, for example, moving from a state shown in FIG. 18(a) to one shown in FIG. 18(b), does actually not stop in the latter state, but stops displaced by .DELTA.x' in the Y2' direction as shown in FIG. 21. The reason is that first in single phase excitation, the core C' is obliged to have a residual magnetism .phi.r' (FIG. 21) even when the current conduction through the coil A' is cut off, and the residual magnetism .phi.r' produces an unbalance between thrust force caused by the magnetic pole P1' and that by the magnetic pole P3', the unbalance then causes the movable member I' to be subjected to a force in the arrow Y2' direction. Likewise, when the movable member I' moves in the arrow Y2' direction, it stops displaced by .DELTA.x' shown in the same figure in the arrow Y1' direction. Namely, it has its different stop positions which depend on the direction of movement prior to stopping. In addition, in two phase excitation, residual magnetism .phi.r' remains when the current flowing through the coil A' is reversed; the residual flux .phi.r' by reacting with the reversed current through coil A' provides a back magnetomotive force which acts against the formed going magnetomotive force after the current inversion so as to cause unbalanced thrust forces. Prior linear stepping motors encounter the displacement 2.DELTA.x' of several .mu.m due to the hysteresis of several micrometers, and hence suffer from accuracy when highly accurate positioning is desired as in cases wherein in particular a magnetic head in a floppy disk drive is driven.
Furthermore, a magnetic material reduced in residual magnetism may be employed for the cores C' and D' in order to remove the hysteresis described above, but the material is very expensive and is incapable of completely eliminating such hysteresis. Moreover, it may also be considered to magnetically anneal the cores C' and D', but its execution requires much labor.